a) Field of the Invention
The invention is directed to a method for the spectrometric determination of the oxygen saturation of blood in the presence of optical disturbance variables such as those also presented by pigmented and light-scattering surrounding tissue and/or also the vascular wall itself. This problem of determining the oxygen saturation of blood while excluding as far as possible the influence of these factors affecting measurement accuracy occurs particularly in noninvasive, in-vivo or in-vitro examinations of blood vessels located in front of, behind, or in said pigmented and scattering tissue, for example, in the examination of blood vessels of the ocular fundus or other tissue areas in the body such as the skin and organs that are accessible by endoscopy.
b) Description of the Related Art
It is generally known that the absorption spectrum of the red blood pigment hemoglobin changes with oxygen saturation (e.g., van Assendelft, O. W., Spectrophotometry of heamoglobin derivatives, Assen: Royal Vangorcum, 1970). Accordingly, it is possible to determine the oxygen saturation of a hemoglobin sample by comparing the spectrum of the sample to the spectra of completely oxygenated and completely reduced hemoglobin.
Recent work in oxymetry in the ocular fundus using the Lambert-Beerschen law, i.e., taking only absorption into account, has been published by Smith et al. and others (Smith, M. H., Denninghoff, K. R., Lompado, A., Hillman, L. W., Effect of multiple light paths in retinal vessel oxymetry, Appl. Opt. 39, 2000, 1183-1193). Numerous patented arrangements and methods are based on this principle (e.g., U.S. Pat. No. 4,485,820; U.S. Pat. No. 5,119,814; U.S. Pat. No. 5,308,919; U.S. Pat. No. 4,253,744; U.S. Pat. No. 4,305,398; U.S. Pat. No. 5,776,060; U.S. Pat. No. 5,935,076; DE 199 20 157 A1; U.S. Pat. No. 5,318,022).
However, the hemoglobin does not exist in isolation in in-vivo measurement, but is enclosed in the erythrocytes. The scattering of light on the erythrocytes has a considerable influence on the extinction spectrum of the blood. However, based on the findings of the multiple scattering theory of Twersky (Twersky, V., Absorption and multiple scattering by biological suspensions, J. Opt. Soc. Amer. 60, 1970, 1084-1093), the influences of scattering and absorption can be separated. On this basis, Pittman and Duling describe a method for determining the oxygen saturation in whole blood from measurements taken in transmission at a wavelength of 555 nm and at isosbestic points at 522 nm and 546 nm (Pittman, R. N., Duling, B. R., A new method for the measurement of percent oxyhemoglobin, J. Appl. Physiol., 38, 1975, 315-320). This method has been used by Delori to determine oxygen saturation in retinal vessels (Delori, F. C., Noninvasive technique of oximetry of blood in retinal vessels, Appl. Opt. 27, 1988, 1113-1125).
However, investigations by Hammer at al. (Hammer, M., Leistritz, S., Leistritz, L., Schweitzer, D., Light paths in retinal vessel oxymetry, IEEE Trans Biomed Eng 48 (5), 2001, 592-8) show that the reflection spectra measured on retinal vessels are influenced not only by the absorption of the hemoglobin and the scattering in the blood and in the tissue surrounding the vessels, but also by the melanin located in the retinal pigment epithelium and in the choroid. This is also true of vessels in the skin or other pigmented organs.
Correcting falsification of the hemoglobin spectra by means of other chromophores and correction for spectroscopic oxymetry have been attempted in previous literature by scaling the spectra measured on a vessel to measurements next to the vessel (e.g., DE 199 20 157 A1; U.S. Pat. No. 5,935,076; Delori, F. C., Noninvasive technique for oximetry of blood in retinal vessels, Appl. Opt. 27, 1988, 113-1125; Schweitzer, D., Hammer, M., Kraft, J., Thamm, E., Königsdörffer, E., Strobel, J., In Vivo Measurement of the Oxygen Saturation at the Normal Human Eye, IEEE Trans. Biomed. Eng. 46, 1999, 1454-1465). However, this approach does not take into account the extremely complicated relationships (Hammer, M., Leistritz, S., Leistritz, L., Schweitzer, D., Light paths in retinal vessel oxymetry, IEEE Trans Biomed Eng 48 (5), 2001, 592-8) of the beam propagation in the blood vessel and the tissue surrounding it.
The exact light propagation in the biological tissue cannot always be fully described physically. Even efforts to recreate these processes for eliminating disturbances in the most comprehensive and realistic manner possible (DE 199 20 157 A1) and to model the optics of the living or dead biological tissue surrounding the blood vessel (DE 44 33 827 A1) have not led to more exact measurements than the methods mentioned above, which are already relatively time-consuming and require intensive computation. In view of this, the methods are only conditionally suitable especially for routine examinations and screening examinations. In particular, the determination of oxygen saturation at every point of a two-dimensional, graphic recording, which is important for clinical practice, requires a method that is fast on the one hand and that compensates for optical and spectrometric disturbances due to the tissue environment on the other hand.